The present invention provides a testing method and system for a telecommunications network, and in particular a method and system that can analyse measured test data and make one or more inferences regarding the nature of the network under test.
The access network of telecommunications networks conventionally comprises copper pairs, which are used to connect customer premises to a local exchange. Voice and data signals are sent to and from the customer premises to the local exchange and onwards to the core network. Data signals are often DSL (Digital Subscriber Line) signals which allow data signals to be multiplexed in the higher frequency regions beyond the portion of the frequency spectrum which is used for the voice signal. Even though there is a trend for optical fiber to be installed into the access network in order to be able to provide data at even greater speeds, it is clear that legacy copper access networks will still be used for a significant period of time.
FIG. 1 shows a schematic depiction of a conventional access network 100 which comprises local exchange 110, a plurality of primary nodes 120 (also known as primary cross-connection points [PCPs]) and a plurality of secondary nodes 130 (also referred to as distribution points (DPs). The local exchange 110 is connected to each of the primary nodes 120 via exchange cables 115. Conventionally, in the United Kingdom, a primary node takes the form of a green roadside cabinet. Each of the primary nodes 120 is connected to one or more secondary nodes 130 via a distribution cable 125. The secondary nodes are often located at, or near to, a telephone pole. Each of the secondary nodes is connected to one or more customer premises 140 via a drop cable 135. The drop cable may be routed to the customer premises on an overhead cable route via a telephone pole or via an underground cable route, for example within a duct. For the sake of clarity, FIG. 1 shows only the connections from one of the primary nodes to the secondary nodes: it should be understood that all of the primary nodes will be connected to a number of secondary nodes
The exchange cables are jointed to the distribution cables at the primary nodes and the distribution cables are connected to the drop cables at the secondary nodes. Furthermore, there may be further inline cable joints located between the local exchange and a primary node, or between a primary node and a secondary node. Poorly made cable joints, water ingress into the housings holding the cable joints and damage to the cable sheaths can all lead to faults occurring on the circuit between the local exchange and a particular customer premises. A fault in a primary node, or in a cable joint located between the local exchange and a primary node may cause a fault that effects multiple customers.
FIG. 2 shows a schematic depiction of the local exchange 110 which comprises telephony switch 112, network testing equipment 114, operational support systems (OSSs) 116 and OSS workstations 118. The exchange cables are connected to the telephony switch which is in turn connected to a core network node 150 via backhaul cable 155. The local exchange may also comprise equipment for routing data to and from the core network (and also to and from the core network of other communications providers) but as this is not relevant to the present invention it is not shown in FIG. 2 for the sake of clarity. The network testing equipment 114 is arranged such that it is able to interface with the exchange cables such that it can send test signals along one or more selected copper pairs. Testing may be carried out for a number of reasons, for example to characterise the performance of the network, to detect conditions that may indicate that faults may occur in the future or in response to customer complaints. In order to test a line it is necessary to disconnect it from the telephony switch so any planned testing is normally performed at a time at which the line is unlikely to be in use and the line will be checked to see if it is active before testing is commenced.
Both the telephony switch and the network testing equipment are in communication with the OSSs 116 which enable the operation and maintenance of the network to be controlled and monitored. One or more workstations 118 may be provided to allow operators to access the OSSs. It will be understood that the OSSs and/or the workstations may not be physically located at each of the local exchanges in a network but may instead be centralised in one or more network management centers which are remote from, but connected to, the local exchange.
As the circuit between the local exchange and customer premises comprises a pair of copper wires, it can be modelled using conventional electrical parameters. FIG. 3 shows a schematic depiction of a line 300 under test. The line is the combination of the exchange cable pair, distribution cable pair and drop cable pair for a particular circuit and comprises A wire 302 and B wire 304. FIG. 3 shows at the local exchange the line 300 has been disconnected from the telephony switch and has been connected to the network testing equipment 114. The line 300 is connected to terminal equipment 142 which is located at the customer premises 140. The terminal equipment may be a network termination module which comprises a socket to allow the connection of a telephone handset and/or DSL modem. Each of the lines 302, 304 has a resistance which depends upon its diameter and the distance from the local exchange to the terminal equipment 142. Each of the wires 302, 304 is coated with an insulating material, which may be a plastic material or paper. The function of the insulating material is to provide insulation between each wire and adjacent wires. Damage to the insulating material or oxidation of the metal of a wire can cause the resistance between two adjacent wires to fall.
The effectiveness of the insulation between wires 302, 304 can be determined by measuring the resistance R1 between the A wire 302 and the B wire 304 and the resistance R2 between the B wire 304 and the A wire 302. The resistances R1 and R2 may be different because of rectification as indicated by diodes D1 and D2. For a circuit in good condition, the resistances R1 and R2 are high, for example greater than 1 MΩ. Damage to the insulating material or oxidation will cause the resistances R1, R2 to fall by an amount which depends upon the severity of the damage or oxidation. If the insulating material is totally destroyed so that the A and B wires are physically touching each other, the values of resistances R1, R2 will depend upon the distance between the network testing equipment and the point of damage but will typically lie in the range 0 to 1500Ω. Oxidation can result in wires effectively touching each other.
Only the A and B wires 302, 304 of the line 300 being tested are disconnected. In the other lines which will be contained within a cable, the bias voltage of 50 volts is applied between the A wire and the B wire. In FIG. 3, the A wires of the other lines are collectively shown by a wire 310 which is connected at the switch 10 to earth. The B wires of the other lines are collectively shown by a wire 312 connected at the switch to a potential of −50 volts.
If the insulating material separating the A wire 302 or the B wire 304 from one of the adjacent A or B wires becomes damaged, or if one of the wires suffers oxidation, current may flow. The effectiveness of the insulation between the A and B wires 302, 304 and adjacent A and B wires can be determined by measuring the resistance R3 between A wire 302 and adjacent A wires 310, the resistance R4 between the A wire 302 and adjacent B wires 312, the resistance R5 between the B wire 304 and adjacent A wires 310, and the resistance R6 between the B wires 304 and adjacent B wires 312.
For a good circuit, the resistance values for R3, R4, R5, R6 are high, for example greater than 1 MΩ. Damage to insulating material may cause one or more of the resistances R3, R4, R5, R6 to fall by an amount which depends upon the severity of the damage. If the insulating material between the A wire 302 or the B wire 304 and an adjacent wire is totally destroyed so that the two wires are physically touching each other, the resistance between the two touching wires will depend upon the distance between the network testing equipment and the point of damage but will typically lie in the range 0 to 1500Ω. Oxidation can also result in two wires effectively touching each other.
The A and B wires 302, 304 and the insulating material between them act as a capacitor. In FIG. 3, the capacitance between the A and B wires is shown as having a value C1. The value of the capacitance between the A and B wires of a line will depend upon the length of the line. A break in the line 300 will reduce the value of capacitance C1 as measured from the network testing equipment. FIG. 3 also shows the capacitance C2 between the A wire 302 and earth and the capacitance C3 between the B wire 304 and earth.
Periodically, for example each night, the network test equipment measures the resistances R1, R2, R3, R4, R5, R6 and the capacitances C1, C2, C3 for each terminating line of the access network 100. The test equipment also checks if there is terminal equipment connected to the end of the line. The terminal equipment has a standard capacitance value. When terminal equipment is connected, the value of its capacitance is subtracted from the capacitance value measured by the test equipment to obtain the capacitance C1. Other measurements may be made, for example measuring the response to the application of a pre-determined voltage, insertion loss at one or more pre-determined frequencies, etc.
For each terminating line, the results of the tests are sent to the OSS such that the results can be stored in a database. If a fault condition is identified then the necessary repair can be scheduled as required. The possible faults include disconnection, short circuit, a fault battery voltage, an earth fault and low insulation resistance. This allows for trends in parameters to be analysed and if the trends indicate that a fault condition is likely to occur in the near future then preventative maintenance can be scheduled. By associating the results with the line with which they are associated then it is possible to correlate faults with particular nodes and/or cables, which assists in the identification of the location of a particular fault.
It is also possible for an engineer to initiate a test, either at a node or at a cable joint. The engineer has a test set which can implement the functionality of the network testing equipment and can measure the various resistance and capacitance values. The test set may be implemented in a laptop computer or similar portable device. In this case, as well as storing test data for later transmission to the OSSs, the test set may analyse the measured data and provide an indication to the engineer of the various parameters along with any likely fault condition or status.
WO2007/050001 discloses a method of determining the transmission properties of a telecommunication transmission line. A signal is sent over the transmission line and the received signal is analysed using a fast Fourier Transform. The results of this analysis include the line capacitance, resistance, inductance and conductance.
US 2003/235274 discloses a method for testing a telephone network to determine whether a line can support high speed data services. A mapping is created between low frequency measurements and average loop loss over a high frequency range. The average loop loss is then used to compute the equivalent working length of a line, which indicative of the ability of a line to support high speed data services.
According to a first aspect of the present invention there provided a method of testing a transmission line in a communications network, the communications network comprising a local exchange and a plurality of transmission lines connected to the local exchange, the method comprising the steps of: i) measuring a value for each of a plurality of transmission line parameters; ii) determining an estimate for the length of the transmission line for each of the plurality of transmission line parameters based on the associated value measured in step i) and a pre-determined further value associated with each of the plurality of transmission line parameters; iii) determining a weighted average transmission line length based on the plurality of transmission line length estimates determined in step ii); and iv) inferring the condition of the transmission line based on the weighted average determined in step iii) and the plurality of transmission line length estimates determined in step ii).
In a first embodiment of the present invention, in step iv) the ratio of the estimated transmission line length to the weighted average transmission line length is determined for each of the plurality of transmission line parameters and the condition of the transmission line is inferred in accordance with the plurality of ratios. The condition of the transmission line may be inferred as being acceptable if each of the plurality of ratios determined in step iv) are less than a first predetermined value. Alternatively, it can be inferred that the transmission line may have an unacceptable condition in the future if each of the plurality of ratios determined in step iv) are greater than the first predetermined value but less than a second predetermined value. Furthermore, it can be inferred that the transmission line has an unacceptable condition if each of the plurality of ratios determined in step iv) are greater than the second predetermined value.
In a second embodiment of the present invention, step iv) comprises the further steps of: a) for each of the plurality of transmission line parameters, determining a data point based on the weighted average transmission line length and the transmission line length estimate associated with each of the transmission line parameters; b) comparing the data point determined in step a) against a pre-determined distribution of data points; and c) inferring the condition of the transmission line based on the comparison made in step b).
In step c), the transmission line may be determined to have an acceptable condition if each of the data points determined in step a) is less than an upper bound and greater than a lower bound, the upper and lower bounds being determined from the distribution of data points. Alternatively, in step c), the transmission line can be determined to have an unacceptable condition if one or more of the data points determined in step a) is greater than the upper bound or lower than the lower bound.
The upper bound and the lower bound may comprise a pre-determined proportion of the data points which comprise the distribution of data points. The upper bound and the lower bound may be defined in accordance with a statistical parameter derived from the pre-determined distribution of data points; this statistical parameter may be the coefficient of variance of the pre-determined distribution of data points.
According to a second aspect of the present invention there is provided an apparatus comprising a processing unit, memory means and data storage means, the apparatus being configured, in use, to perform a method as described above. The apparatus may comprise a portable network testing apparatus.
According to a third aspect of the present invention there is provided a data carrier for use in a computing device, the data carrier comprising computer executable code which, in use, performs a method as described above.